Currently, the deployment of heterogeneous networks (HetNets) is viewed as one of the most cost efficient deployment strategies for wireless communication systems in addressing the growing traffic demands and the expectation for higher data rates. Typical cellular networks today are characterized by non-uniform user and traffic distributions. HetNets complement the macro networks with low power nodes (LPNs) of a diverse nature, such as micro, pico, and femto base stations or relay nodes, which can achieve significantly improved capacity and high data rates. The resulting fragmented multi-layer architecture HetNet is illustrated in FIG. 1A, showing a macro cell 104 as part of a communication network 102, with a first node, or base station 106, and with second node LPNs, or base stations, 108-112.
In heterogeneous networks, there are various types of base stations, each of which can be associated with differing cell sizes. For instance, large base stations, such as macro cell base stations 106, are typically installed on masts, rooftops and other existing structures. Macro cell base stations 106 normally have power outputs on the order of tens of watts and, thus, provide large cell coverage. Small base stations, such as micro, pico and femto cell base stations 108-112 are LPNs, which are commonly designed for residential or small business environments. The power outputs for these small base stations 108-112 are normally less than a watt to a few watts, which results in a small cell coverage range.
HetNets are less susceptible to the strains on signal power (due to the increase in distance from the transmitting point) and are well known to defy the inverse square law of distance by moving the Base Station (BS) closer to users and providing similar Quality-of-Service (QoS) throughout the cell area. Thus, HetNet deployments possess an inherent capability to address the limitations implied by channel capacity and to provide a uniform user experience throughout the cell area, irrespective of user location. The potential of HetNets to bring gains in coverage and capacity are accomplished because:                Moving the BS closer to users results in better radio link conditions, which in turn leads to higher data rates for users connected to the low power nodes 108-112.        LPN cells 114-118 provide access to the users previously handled by the macro layer, thus reducing the load from the macro cell 104 (called “macro offloading”). This results in higher availability of resources and thus higher data rates for the users connected to macro base stations 106.        HetNet deployments generally tend to provide uniform data rates within a given area 104.        
Within 3GPP Long Term Evolution Advanced (LTE-A), three types of LPNs are specified within the 3GPP TR 36.814 specification standard according to their respective access rights. Open Access (OA) LPNs provide that any User Equipment (UE) 120, 122, 124, and 132 can associate with the Open Access LPN if the LPN cell is the one providing the best signal quality to the UE. As used herein, the terms LPN and LPN cell represent any type of small range, or low power, cell—e.g., micro, pico or femto cells 114-118. The terms LPN, micro, pico and femto cells are used interchangeably herein. Closed Subscriber Group (CSG) LPNs 108-112 provide additional privacy to the user as control is granted to the end user by letting the cell owner authorize access to the LPN 108-112 and configure the set of UEs 120, 122, 124, and 132 to be provided service by the LPN 108-112, called authorized UEs. Thus, the cell owner selects a set of UEs and configures the CSG LPN to limit access only to them. Hybrid Access (HA) LPNs are a composite of OA and CSG modes; which by default operate in CSG mode while switching into OA mode at certain instances, thus providing service to certain non-authorized UEs temporarily, e.g., visitors within a given home, etc.
The access nature of the LPNs 108-112 has different implications on system performance based on the tradeoff between interference and increased capacity. The incorporation of LPNs 108-112 within the macro cell 104 can cause interference to the macro UEs 126-130 that are in close proximity to the LPNs 108-112 but are associated with the macro cell 104. OA tends to provide higher gains in system capacity and increase UE throughput, while causing less interference to the nearby UEs. However, the presence of CSG-type LPNs 108-112 within the macro cell 104 have a tendency to accentuate the interference scenarios and to enhance the cell edge effect in the macro cell area. As the CSG cells 114-118 provide access only to a limited number of UEs 120, 122, 124, and 132, the nearby macro UEs 126-130 experience severe interference from the CSG cells 114-118. In particular, the presence of CSG cells 114-118 causes significant interference problems for the macro UEs 126-130 and results in very low Signal to Interference-plus-Noise Ratio (SINR), UE throughput, and a reduced system capacity.
The presence of CSG cells 114-118 intensifies the serving conditions in the macro cell area 104 and, as a result, more UEs experience service outages. The prime reason for the cause of degraded system performance for the CSG scenarios is the interference resulting due to limited service provisioning constraints of the CSG cells 114-118. As the CSG cells 114-118 only provide access to a limited number of UEs (called authorized UEs) 120, 120, 124, and 132, the unauthorized macro UEs 126-130 undergo severe interference from the CSG transmission, which results in reduced performance, but cannot access the CSG cell 114-118 despite the relative proximity of the CSG base station 108-112. By contrast, in OA scenarios, if a UE is operating close enough to an OA base station to experience significant interference from the OA base station, the UE will typically be able to access the corresponding OA cell and use the OA base station as its serving base station.
FIG. 1B illustrates major interference scenarios that can be observed in a HetNet deployment. For example, the downlink transmission signals 138 and 148 from the CSG LPNs 112 and 110 to the macro UE 130 causes interference between the macro UE 130 and its macro BS 106. Further, with respect to the uplink (UL) transmission signals, macro UEs associated with the macro cell, when located close to a CSG cell, cause high UL interference for the UEs connected to the CSG cell and transmitting in the UL.
CSG cells 114-118 have high tendencies to impact the performance of nearby UEs 126-130 that are connected to the macro cell 104. A major source of interference in HetNet deployments featuring CSG cells is the transmission of Cell-specific Reference Signals (CRS) from the CSG cells. FIG. 2 illustrates the exemplary downlink SINR of macro UEs for different UE types. The Reference Case refers to a macro only scenario without any CSG LPN cells in the macro cell area 104, and the CSG Case refers to the situation where there are, for example, ten CSG LPNs in the macro cell. Further, for example, 20% of the UEs in the macro cell are connected to the CSG cells. Under this scenario, as shown in FIG. 2, if the UEs are not capable of cancelling interference resulting from the CRS signals, the average SINR of the macro UEs is observed to suffer significant reduction. For example, as shown in column 204, the SINR for the macro UEs that lack CRS interference cancellation (IC) capability is 0.39 dB. In contrast, if the macro UEs are capable of cancelling CRS signal interference, the CSG cells tend to improve the SINR of the UEs by offloading the macro cell, as shown in column 206 with the SINR of 6.4 dB. This indicates that a major part of interference in CSG-featured HetNet deployments is caused due to the transmission of CRS signals from the CSG BSs. Since these CSG cells provide service to a limited number of UEs, the Physical Resource Block (PRB) utilization in these cells is very small (e.g., typically less than 10%, even for a heavily loaded traffic scenario), and thus the transmission on the Physical Downlink Shared Channel (PDSCH) is less frequent. Hence, the interference is mainly caused by the CRS signals which are transmitted (in each sub-frame) by all BS in the downlink (DL).
Since the CRS interference cancellation is a relatively new concept to be introduced in UEs of Rel-11, the UEs of previous releases lack this functionality. Thus, for HetNet deployments featuring CSG cells, there is a need to devise and implement efficient Inter-Cell Interference Coordination (ICIC) techniques to limit the negative performance impact on macro UEs under the influence of interference from the CSG cells.
The current techniques aiming to overcome the problem of interference due to the transmission of neighboring cells' CRS can be characterized into Network Assisted (NA) or UE-Implemented (UE-I) solutions. In the NA solutions, static or semi-static (pre-settled policy, triggered by an event) procedures are implemented among the BSs to perform ICIC. In contrast, the UE-Implemented procedures mainly advocate the cancellation of interference due to CRS at the UE.
One NA technique is Enhanced Inter-Cell Interference Coordination (e-ICIC) using Almost Blank Sub-frames (ABS). The ABS technique controls the macro cell, the LPNs, or both to limit the muting of transmissions at distinct intervals to avoid interference. The macro cells mute their transmissions in alternate sub-frames to protect the UEs associated with the pico cells from the interference caused by the macro. However, to perform channel measurements and to maintain compatibility among the UEs of different releases, CRS signals are transmitted in all sub-frames (including the muted sub-frames). This technique may be useful for deployments featuring OA cells, where the interference experienced by the UEs is dominated by the transmissions on the Physical Downlink Shared Channel (PDSCH). However, for the circumstances where CSG cells are deployed in the system, muting of the macro cell or the CSG cells is not an acceptable solution since most of the traffic (such as more than 95%) in such a scenario is being served by the macro cell (i.e. high macro PRB utilization). This means that muting transmissions of the macro BS is not desirable because it will result in excessive scheduling delays and lead to congestion in the macro cell. Further, muting transmissions of the CSG cells does not reduce the DL interference to macro UEs, since the interference is mainly caused by the transmission of CRS, and in the ABS technique, CRS are transmitted in all sub-frames (even in the muted ones).
A UE-Implemented technique is known as Cell-specific Reference Signals -Interference Cancellation (CRS-IC). CRS-IC is an Interference Cancellation (IC) mechanism implemented in the UE with the goal of minimizing the interference from CRS with use of appropriate signal processing. As the CRSs are transmitted on pre-determined intervals with a well determined format, the UEs can reliably estimate, or obtain these signals from neighbor cells, and thus can perform the interference cancellation without the need for any strict coordination mechanisms between cells. However, implementing CRS-IC within UEs in general is a complicated task which increases the device cost and results in increased UE energy consumption. Furthermore, removing the interfering signals from each transmission takes away part of the useful signal, resulting in low SINR. Moreover, cancellation of interference from the CRS under CRS-IC is problematic due to estimation errors. Further, since CRS-IC is a relatively new concept to be introduced in UEs of Rel-11, the UEs of previous releases lack this functionality.
Accordingly, there is a need to reduce interference in UEs that may be caused by data and/or reference signals from, for example, UE devices connected to a neighbor cell in a HetNet deployment featuring CSG cells.